One of the rules for floats positioning states that: “The outer top of a floating box may not be higher than the outer top of any block or floated box generated by an element earlier in the source document.” This is rule 5 in the W3C specification.
At the top of this page there is a sequence of sequentially numbered floated boxes (with the same height and different widths.) If the width of the browser window is reduced enough, the boxes wrap on multiple 'rows'. According to the mentioned rule, a higher-numbered box should never be positioned higher than a lower-numbered one.
Opera 7 doesn't honor this rule. Reduce slowly the browser width and check how the boxes wrap. Op7 places each box in the highest position possible (inside the gray wrapper.)
For example when the gray wrapper is 468px wide, then Op7 displays this sequence:
1 2 4 6
3 5
while the correct positioning is:
1 2
3 4 5 6
The example at the end of the page uses a fixed width container to show this particular case.
Tested in Op7 up to 7.54u2. Other browsers are correct here, including IE5+ (Win & Mac), Moz, Saf, and Opera 6! The problem has been corrected in Opera 8.